Filters. For those of us with ye olde fashioned film-using cameras : )
I once had a book of filters. The stuff you could do with them, no computers necessary, was rather awesome. Lots of neat effects; blurring for motion, half-blurs, colours, etc. Also avoiding polarisation. I have such a filter, but I don't think I'm using it right because when taking photos of shop windows I still get reflections, which I'm trying to remove. Also water.
They say birds who eat from water can polarise light with their eyes, allowing them to see the fish in the water under the surface.
I was once introduced to a type of film whose name I've forgotten, but it was sensitive to heat or infrared light. It was expensive, had to be stored in the fridge and was great for taking portraits, making people look like they glow a bit : )